1. Field of the Invention
The present invention is directed to coating compositions based on polyisocyanates and aldimines which have improved pot lives without a corresponding increase in dry times due to the presence of compounds which are catalysts for the reaction between isocyanate groups and hydroxyl groups.
2. Description of the Prior Art
Coating compositions which may be cured at room temperature are known. One-component coating compositions contain fully reacted polyurethanes as the binder. These compositions have the advantage that they are available as fully formulated systems which may be directly applied to suitable substrates without any preliminary steps except for mild stirring. Disadvantages of these systems are that large amounts of organic solvents are needed to reduce the viscosity of fully reacted, i.e., high molecular weight, polyurethanes. The coating compositions are cured by evaporation of the solvent which is objectionable from an environmental perspective. In addition, in order to solubilize the polyurethanes in organic solvents, they must be essentially linear polyurethanes. While such polyurethanes possess properties which are suitable for many applications, they do not provide certain properties, e.g., solvent resistance which may be obtained from crosslinked polyurethanes.
Two-component coating compositions are also known. These compositions come in two containers. The first contains a polyisocyanate, while the second contains an isocyanate-reactive component, generally a polyol. The components are not mixed until they are ready to be used. One advantage of these compositions is that because the components are not pre-reacted to form a high molecular weight polymer, a suitable processing viscosity can be achieved without the need for large amounts of organic solvents. In addition, higher functional components can be used to obtain highly crosslinked coatings which possess properties which surpass those possessed by one-component coatings.
The disadvantages of these compositions is that they cannot be applied without a preliminary mixing step in which it is critical that the components are mixed in the right proportions. In addition, special metering and mixing equipment is needed to conduct this process on a commercial scale. If the components are mixed in the wrong proportions, then the properties of the resulting coatings can be substantially affected. In addition, after the components are mixed they must be used in a timely fashion. If not, they continue to react until an unusable solid is obtained.
Coating compositions which possess the advantages of the known one- and two-component coating compositions without possessing their disadvantages have been disclosed in copending application, Attorney's Docket No. MD-93-108-IC. Even though coatings prepared in accordance with the copending application possess many desirable properties, further improvements are needed in the pot lives of the compositions. The viscosity of these compositions increases too rapidly prior to being applied to a substrate and cured.
Accordingly, it is an object of the present invention to provide increased pot lives without significantly increasing the dry times of the coating compositions and without altering any of the other desirable properties of the compositions.
Surprisingly, this object may be achieved with the coating compositions of the present invention which contain polyisocyanates and aldimines and also certain tin compounds to be described hereinafter. It is surprising that an increase in the pot life can be obtained by incorporating these tin compounds because they are generally regarded as catalysts for polyisocyanate polyaddition reactions and, thus, would be expected to shorten the pot life of the coating compositions.
U.S. Pat. Nos. 3,420,800 and 3,567,692 disclose coating compositions containing polyisocyanates and either aldimines or ketimines. However, these patents do not teach the use of tin compounds to increase the pot life. In addition, U.S. Pat. No. 5,243,012 teaches that tin compounds may be used to increase the pot life of coating compositions containing polyisocyanates and polyaspartic acid derivatives that contain secondary amino groups.